RFR: 8265938: C2's conditional move optimization does not handle top Phi

[Tobias Hartmann]

C2's CCP optimization sets the type of a PhiNode to TOP because it became dead after loop unrolling. Now since that node is not reachable from the bottom due to an infinite loop at the end of the method, CCP does not enqueue the PhiNode for IGVN. As a result, the dead subgraph is not removed. During loop opts, the conditional move optimization fails on the PhiNode because its type is top. The expected behavior is to simply bail out with "COMPILE SKIPPED: infinite loop (retry at different tier)".

I suggest to fix this by simply handling a TOP Phi in the conditional move optimization code.

This issue was reported by John Jiang (johnsjiang at tencent.com).

Thanks,
Tobias

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Commit messages:
 - 8265938: C2's conditional move optimization does not handle top Phi

Changes: https://git.openjdk.java.net/jdk/pull/3684/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3684&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265938
  Stats: 71 lines in 2 files changed: 70 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3684.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3684/head:pull/3684

PR: https://git.openjdk.java.net/jdk/pull/3684